The Source engine is a 3D game engine developed by Valve Corporation. Its unique features include a large degree of modularity and flexibility, an artist-driven, shader-based renderer, accurate lip sync and facial expression technology, and a powerful, efficient and completely network-enabled physics system.
This engine has a lot of pros and cons. One one hand, it's very easy to get into and it can look pretty damn good in the right hands. But on the other hand, you got a 7 year old tool set with mostly command line tools that Valve breaks when ever they get bored. And depending on the version of Source you use(2006, 2007, 2009(no code), AlienSwarm, Portal2(no code)), you might get stuck with half working features(like shadow mapping in 2007).
The main pro is; the fact that it comes with an entire game(2006/2007).
Main con is; half working tools that love to crap out for no apparent reason.
If the tools were in decent shape, and if they would release more fully working(AS has no shader support) codebases. Then I would give it a 10/10 and wouldn't be in the process of learning the UDK.